The invention relates to a process for the production of powder comprised of fine spherical particles of metallic nickel. More particularly, the invention relates to a process for the production of powder comprised of fine spherical particles of metallic nickel which have an average particle size of 0.05 to 10 xcexcm, preferably 0.1 xcexcm to several microns, and most preferably 0.1 to 1 xcexcm and a narrow particle size distribution, which are comprised of high crystallinity metallic nickel. Accordingly, the powder of fine spherical particles of metallic nickel produced by the invention is suitable for use as inner electrodes of laminated ceramic capacitors.
With recent progress of development of small-sized electronic parts having large capacity, more small-sized laminated ceramic capacitors with large capacitance are demanded. A laminated ceramic capacitor is produced as follows. Dielectric green sheets are prepared by using powder of ceramic dielectrics such as barium titanate and a binder such as polyvinyl butyral. A paste containing powder of a noble metal such as palladium or platinum for inner electrodes is coated on the green sheet by a printing process and dried. Then, a plurality of the green sheets are placed in layers and heated so that they are bound to each other and the dielectric sheets and the inner electrodes are laminated alternately.
The resulting laminated product is then cut to an appropriate size and calcined at a temperature of about 1300xc2x0 C. so that the inner electrodes and ceramic dielectric are sintered while removing the binder used therefrom. Finally, outer electrodes are formed with, for example, silver.
As clear from the foregoing, it is necessary that the metal for use as inner electrodes does not melt at a temperature at which the ceramic dielectric used sinters, and besides, it is not oxidized. Accordingly, expensive noble metals such as platinum or palladium are used thus far as inner electrodes as mentioned above to make laminated ceramic capacitors expensive as well.
Thus, recently, a number of studies have been made to put inexpensive laminated ceramic capacitors to practical use in which a base metal, nickel, is used as inner electrodes in place of the conventional expensive laminated ceramic capacitors in which platinum or palladium is used as inner electrodes. However, there is a problem here.
Inner electrodes of laminated ceramic capacitors are limited by the size of the powder of a metal used to form inner electrodes. That is, the inner electrode cannot be thinner than the particle size of the metal powder. The thickness of the inner electrode is usually in the range of one or two microns. Consequently, when a powder of a metal having a particle size of more than one micron is used, the electrode layer becomes non-uniform and there is a fear that continuity defects will arise. In addition, when ceramic dielectric sheets having thereon a coating of a metal to form inner electrodes are placed in layers and laminated, the layer of inner electrode may penetrate through the dielectric layer to cause bad insulation between the dielectric layers.
Therefore, it is demanded that powder of metallic nickel for use as inner electrodes of laminated ceramic capacitors has a particle size in the range of about 0.1 micron to one micron, and besides, it has a narrow particle size distribution, taking packing efficiency of the powder into consideration.
Although a variety of methods have been heretofore proposed to produce fine powder of metallic nickel having such properties as mentioned above, any method already known is apt to produce particles having crystal habit such as of cubic system. Thus, there is disclosed a method in which vapor of nickel chloride is reduced with hydrogen in a gas phase under a low partial pressure of nickel chloride in Japanese Patent Application Laid-Open No. 4-365806, however, this method is costly.
Of course, a method in which various metal compounds including metal oxides are heated and reduced at high temperatures under a pressurized hydrogen gas, as described in a Japanese Patent Laid-Open No. 53-16437, however, any such method has hitherto failed to produce such powder of fine spherical particles of metallic nickel as demanded.
The present inventors have paid attention to oxidation and reduction of nickel salt as a means to produce a powder of fine spherical particles of metallic nickel inexpensively and easily, and have made extensive studies along those lines. As a result, the inventors have succeeded in obtaining a powder of fine spherical particles of nickel carbonate having a uniform particle size, and have found that, if necessary, after oxidizing the fine powder of nickel carbonate to fine powder of nickel oxide, the powder is heated under a hydrogen atmosphere to reduce the powder in the presence of fusion preventive agent, thereby providing a powder of fine spherical particles of metallic nickel in high yields, and have thus completed the invention.
Nickel carbonate is usually in the form of amorphous or non-spherical fine particles, although it is described in Japanese Patent Application Laid-Open No. 2-59432 that fine spherical particles of nickel carbonate can be produced by a method in which an emulsion is used as a reaction field (medium).
Recently, inclusive of the above-mentioned method as described in Japanese Patent Application Laid-Open No. 2-59432, a method in which a W/O emulsion is made use of as a reaction field attracts considerable attention as a means to produce fine spherical particles of inorganic salts having a uniform particle size.
According to such a method, an aqueous solution of water-soluble inorganic salt is added to an organic solvent together with a surfactant, and the resultant mixture is stirred thereby preparing a W/O emulsion, and then a neutralizer (an acid or an alkali is mixed with the emulsion so that a water-insoluble salt is precipitated in the form of fine spherical particles within the fine droplets of the solution.
However, according to such a method in which a W/O emulsion is made use of as a reaction field, the emulsion is easily broken not only on account of a neutralizer used such as an acid or an alkali but also on account of salts by-produced when water-insoluble nickel salts are formed. Accordingly, it is difficult to maintain the reaction field stable throughout the reaction and hence it has been difficult to obtain fine spherical particles of nickel salts having a uniform particle size by a method which makes use of emulsion as a reaction field.
In addition, even if fine spherical particles of nickel salts are obtained, the particles fail to retain their spherical form in the course of oxidation or reduction, for example, so that fine spherical particles of metallic nickel have not been obtained hitherto successfully.
On the other hand, in particular, when powder of metallic nickel is produced by heating and reducing nickel compounds in a hydrogen atmosphere, the generated particles of metallic nickel have tendency to fuse together so that it is difficult to produce fine spherical particles of metallic nickel having a particle size of 10 microns or less in good yields.
The present invention has been completed to solve the various problems as mentioned above involved in the production of fine spherical particles of metallic nickel.
Therefore, it is an object of the invention to provide a process for the production of powder comprised of fine spherical particles of metallic nickel which have an average particle size of 0.05 xcexcm to 10 xcexcm, preferably 0.1 xcexcm to several microns, in particular, preferably 0.1 to 1 xcexcm, and a narrow particle size distribution and which are comprised of high crystallinity metallic nickel, so that they are suitable for use as inner electrodes for laminated ceramic capacitors, in good yields while preventing particles of metallic nickel from fusing together.
The invention provides a process for the production of powder comprised of fine spherical particles of metallic nickel having an average particle size of 0.05 to 10 xcexcm, which comprises:
a first stage of dissolving nickel carbonate and/or nickel hydroxide represented by the general formula (I)
Ni(CO3)x.(OH)y
wherein x and y are numerals which satisfy the conditions: 0xe2x89xa6xxe2x89xa61.5 and 0xe2x89xa6yxe2x89xa63, respectively, in aqueous ammonia or in an aqueous solution of ammonia and at least one selected from the group consisting of ammonium carbonate, ammonium hydrogencarbonate, a carbonate of an alkali metal and a hydrogencarbonate of an alkali metal to prepare an aqueous solution of a nickel salt; and converting the aqueous solution of a nickel salt to a W/O emulsion containing droplets of the aqueous solution in a non-aqueous medium and then removing volatile components including ammonia from the droplets to form precipitates of nickel carbonate in the droplets, thereby providing fine spherical particles of nickel carbonate; and
a second stage of heating the particles of nickel carbonate in the presence of a fusion preventive agent that is a compound of at least one element selected from the group consisting of alkaline earth elements, aluminum, silicon and rare earth elements in an amount of 0.01% by weight to 30% by weight in terms of oxides based on the weight of the nickel carbonate in an atmosphere of hydrogen, thereby reducing the particles of nickel carbonate to particles of metallic nickel.
The invention provides a further process (second process of the invention) for the production of powder comprised of fine spherical particles of metallic nickel having an average particle size of 0.05-10 xcexcm, which comprises:
a first stage of dissolving nickel carbonate and/or nickel hydroxide represented by the general formula (I)
Ni(CO3)x.(OH)y
wherein x and y are numerals which satisfy the conditions: 0xe2x89xa6xxe2x89xa61.5 and 0xe2x89xa6yxe2x89xa63, respectively, in aqueous ammonia or in an aqueous solution of ammonia and at least one selected from the group consisting of ammonium carbonate, ammonium hydrogencarbonate, a carbonate of an alkali metal and a hydrogencarbonate of an alkali metal to prepare an aqueous solution of a nickel salt; and converting the aqueous solution of a nickel salt to a W/O emulsion containing droplets of the aqueous solution in a non-aqueous medium and then removing volatile components including ammonia from the droplets to form precipitates of nickel carbonate in the droplets, thereby providing fine spherical particles of nickel carbonate; and
a second stage of heating the particles of nickel carbonate in an oxidative atmosphere to provide fine spherical particles of nickel oxide; and then heating the particles of nickel oxide in the presence of a fusion preventive agent that is a compound of at least one element selected from the group consisting of alkaline earth elements, aluminum, silicon and rare earth elements in an amount of 0.01% by weight to 30% by weight in terms of oxides based on the weight of the nickel oxide in an atmosphere of hydrogen, thereby reducing the particles of nickel oxide to particles of metallic nickel.
In brief, according to the second method of the invention, fine spherical particles of nickel carbonate are produced in the first step in the same manner as the first process, but in the second step of the second process, the nickel carbonate is then first thermally decomposed to fine spherical particles of nickel oxide, and then the particles of nickel oxide are reduced to provide fine spherical particles of metallic nickel.